Automatic electric kettles and hot water jugs are conventionally provided with two thermal controls, namely an element protector control adapted to switch off the supply of power to the electric heating element of the appliance in a sensed overtemperature situation resulting from the appliance being switched on empty for example, and a steam control for reducing or switching off the supply of power to the heating element when the water being heated boils. Such controls generally have their own thermal sensors, commonly a bimetallic element, and the element protector control may further include some form of secondary or back-up protection arranged to operate in the event, however unlikely, of failure of the primary control. An exemplary element protector control is the X1control manufactured by Otter Controls Ltd. and substantially as described in GB-A-2194099 with reference particularly to FIGS. 3A, 3B and 3C thereof, and an exemplary steam control is the J1control manufactured by Otter Controls Ltd. and substantially as described in GB-A-2212664 with reference particularly to FIGS. 3A to 3M thereof.
A single sensor electronic control for a water boiling vessel is described in GB-A-2228634. This control utilizes a thermistor and senses element temperature as a function of the electrical resistance of the thermistor and the onset of boiling as a function of the rate of change of the thermistor resistance. However, even with the current state of modern electronics a circuit capable of switching a mains load cannot readily be manufactured at a price competitive with bimetallic controls.
A single sensor control is proposed in GB-A-1 143 834 (Matsushita) but has never been manufactured, so far as we are aware. According to this proposal, a sheathed, insulated, resistance heating element mounted to a head plate, providing for mounting of the element in a kettle or other water boiling vessel, has the heating element proper bent to define a hot return portion which is attached to the element head plate at a location above the general level of the major part of the heating element, and an apertured enclosure is provided on the element head plate in the region where the element hot return portion attaches thereto. The provision of a hot return element portion which attaches to the element head at a level above the major part of the heating element proper was conventional at the time of filing of GB-A-1 143 834, this arrangement ensuring that if a non-automatic kettle is accidentally left to boil dry the hot return part of the element will be exposed first as the water level drops and will consequently overheat thereby providing a thermal signal through the element head to a bimetallic or other control provided on the opposite side thereof. By this means the element can be switched off before any substantial part of the element seriously overheats. By enclosing the hot return part of the element within an enclosure provided with small openings sufficient to admit water to the interior of the enclosure when the kettle is cold, the proposal of GB-A-1 143 834 was that the steam generated within the enclosure on boiling would drive the water from the enclosure whereupon the hot return portion of the element would overheat and cause the bimetallic or other control to operate. In accordance with the arrangement proposed in GB-A-1 143 834, a single bimetallic or other control will operate both when a kettle boils and when a kettle is switched on dry.
As mentioned above, the proposal of GB-A-1 143 834 has not to our knowledge ever been manufactured despite its apparent simplicity and the promised advantage of a single sensor bimetallic control providing both element overtemperature protection and boil sensing. Furthermore, in tests that we conducted we found it impossible with a heating element as described in GB-A-1 143 834 to achieve a sufficient temperature rise on boiling to reliably operate a thermal actuator. Such an actuator, it is to be understood, has to be able to discriminate between the temperatures experienced during normal heating, which may be elevated by lime scale for example, and the temperature experienced when the water boils.